1. Field of the Invention
The present invention relates generally to the testing of asphalt and more specifically to a laboratory asphalt stability test which is able to more thoroughly test the stability of modified asphalt binder than that of the prior art.
2. Discussion of the Prior Art
At present, the most common method for testing asphalt stability is accomplished through the use of the cigar tube test. A cigar type tube is filed with modified asphalt binder and sealed. The sealed cigar type tube is heated in an oven to 163 degrees Celsius for 2 days. The sealed cigar type tube is then frozen and cut into three sections. The top and bottom sections are heated to 163 degrees Celsius for standard testing. A drawback to the cigar tube test is that it does not take agitation of the modified asphalt binder into account. In the field modified asphalt binder is mixed before it is applied. Further, the thermal history of the modified asphalt binder is altered by freezing.
Other asphalt test methods utilize molecular analysis to determine stability. However, these methods are difficult to perform in the field because of their complexity and high cost. They also fail to account for the effects of agitation of the modified asphalt binder.
Accordingly, there is a clearly felt need in the art for a laboratory asphalt stability test which simulates the condition of modified asphalt binder under field conditions by including the effects of agitation and more closely simulating thermal treatment.
The primary objective of the present invention is to provide a laboratory asphalt stability test which simulates the condition of modified asphalt binder under field conditions by including the effects of agitation and more closely simulating thermal treatment.
According to the present invention, an asphalt stability testing vessel includes a container, an external heater, an internal heater, an agitator assembly, a temperature controller, and at least one sampling tube. The external heater is disposed on the outside surface area of the container. The external heater is surrounded by a thermal insulator such as fiberglass. The internal heater is located inside the container and at substantially the bottom thereof. A plurality of baffles are disposed vertically in the container. The agitator includes a motor, a shaft, and at least one propeller. The shaft is pivotally mounted to the top and bottom of the container. A motor is mounted to the top of the container and drives the shaft which has at least one propeller attached thereto. The temperature of the sample may be maintained by the temperature controller. The temperature controller is used to control the thermal output of the internal and external heaters. The top of the container has at least one opening for the insertion of a sampling tube.
The asphalt stability testing vessel is used to prepare modified asphalt binder for numerous stability tests. The stability test is started by heating preferably one quart of modified asphalt binder to 165 degrees celsius. The sampling tubes are pre-heated in an oven. The heated modified asphalt binder is poured into the asphalt stability testing vessel. The sampling tubes are used to extract numerous samples from the top and bottom thirds of the container after the temperature therein has stabilized to 165 degrees celsius. The contents of the sampling tubes are placed into a plurality of silicon molds. The samples are then tested with preferably a dynamic shear rheometer. Other types of rheometers may be used.
The complex shear modulus (G*) is the ratio calculated by dividing the absolute value of the peak-to-peak shear stress by the absolute value of the peak-to-peak strain. The phase angle (xcex4) is the angle in radians or degrees, between a sinusoidally applied strain and the resultant sinusoidal stress in a controlled-strain testing mode, or between the applied stress and the resultant strain in a controlled-stress testing mode.
If either the complex shear modulus (G*) or the phase angle (xcex4) in either the top or bottom samples differ by more than 20 percent, separation has occurred; new samples are tested utilizing the internal heater and high agitation in the asphalt stability testing vessel. If the complex shear modulus (G*) or the phase angle (xcex4) in either the top or bottom samples differ by less than 20 percent, no separation has occurred. If no separation occurs, the values of the top and bottom samples are averaged together and plugged into a degradation equation. If the ratio of the degradation equation is greater than 1.2 or less than 0.8, degradation has occurred; the binder is unstable. If degradation has not occurred a new sample is internally heated and subjected to high agitation, if no degradation has occurred; the binder is stable. If degradation has occurred, a new sample is internally heated without agitation. If degradation occurs, the binder is not stable. If no degradation occurs then the binder is stable only with minimum agitation.
If a sample experiences separation after external heat without agitation, a new sample is internally heated and subjected to high agitation. If separation occurs, the binder is not stable. If no separation occurs, a new sample is subjected to degradation analysis. If no degradation occurs, a new sample is internally heated without agitation. If separation occurs, the binder is stable only at high agitation. If no separation occurs, the binder is stable at minimum agitation.
If degradation occurs, a new sample is internally heated without agitation. If separation occurs, the binder is not stable. If no separation occurs, the values are subjected to degradation analysis. If degradation occurs, the binder is not stable. If no degradation occurs the binder is stable only at minimum agitation.
Microscopic evaluation may also be used to determine the amount of separation in the asphalt in the top or bottom thirds of the container. Under microscopic evaluation, polymer material is brighter than asphalt material. One way of determining separation is to compare the bright polymer spots in the top sample to that of the bottom sample. If the top sample or bottom sample has more bright polymer spots than the other sample, separation has occurred. If the amount of bright polymer spots is the same in the top and bottom samples, no separation has occurred.
Some other properties which are useful in analyzing asphalt samples are loss shear modulus, storage shear modulus, engineering strain, failure stress, failure strain, flexural creep stiffness, flexural creep compliance, logarithmic creep, and viscosity.
Loss shear modulus is the ratio calculated by dividing the absolute value of the peak-to-peak shear stress, by the absolute value of the peak-to-peak shear strain.
Storage shear modulus is the complex shear modulus multiplied by the cosine of the phase angle expressed in degrees. It represents the in-phase component of the complex modulus that is a measure of the energy stored during a loading cycle.
Engineering strain refers to the axial strain resulting from the application of a tensile load and calculated as the change in length caused by the application of the tensile load divided by the original unloaded length of the specimen without any correction for reduction in cross-section.
Failure stress is the tensile stress on the test specimen when the load reaches a maximum value during the test method specified in a particular standard.
Failure strain is the tensile strain corresponding to the failure stress.
Flexural creep stiffness is the ratio obtained by dividing the maximum bending stress in the bending beam rheometer by the maximum bending stress.
Flexural creep compliance is the ratio obtained by dividing the maximum bending strain in the bending beam rheometer by the maximum bending stress.
Logarithmic creep (m value) is the absolute slope of the logarithm of the stiffness curve versus the logarithm of time.
Viscosity is the resistance to flow of a liquid substance.
Accordingly, it is an object of the present invention to provide an asphalt stability test vessel which recreates the conditions of modified asphalt binder in the field.
It is a further object of the present invention to provide a laboratory asphalt stability test which adds the variable of agitation to test modified asphalt binder.
It is yet a further object of the present invention to provide a laboratory asphalt stability test which does not require freezing of the modified asphalt binder.
It is yet a further object of the present invention to provide a laboratory asphalt stability test which may be economically administered.
Finally, it is another object of the present invention to provide a laboratory asphalt stability test which can be easily administered in the field.
These and additional objects, advantages, features and benefits of the present invention will become apparent from the following specification.